1. Field of the Invention
The present invention relates generally to motor speed control and more specifically it relates to a method to signal an encoderless motor controller to maintain low speed motor stability.
2. Description of the Prior Art
It can be appreciated that motor controllers have been in use for years. Motor controllers are used in a wide variety of environments to vary the activation, speed, torque or even the motor shaft position of a motor as required for the application. Manufacturing plants use a multitude of motor controllers for simple and complex operations, for speed control of an air handling fan, to highly precision maneuvers for each step in an automated manufacturing process. Production floor equipment that utilizes computer numerical controls (CNC) is dependent on controllable motors to accurately machine the desired product. For example, a CNC lathe may be used to make machine parts such as a transmission component for a truck transmission. The raw material is clamped to the lathe spindle and a predefined program is initiated by the station operator to manufacture the part by an automatic multi-step process. The program is essentially designed to control the speed of the spindle and the positioning of a number of cutting tools to correctly machine the part. Each movement of the process is provided by a motor and motor controller pair as directed by the specialized program. The accuracy of the finished product is directly dependent on the ability of the motor/motor controller pair to meet design tolerance, to achieve the desired shape.
Alternating Current (AC) variable speed (motor) drives (VSD) are available in three principal types: open loop, encoderless and closed loop that provide increasingly sophisticated command of induction (and permanent magnet synchronous) motors. These VSDs are designed to modulate a power source, where pulse width modulation (PWM) is common, to control small xc2xe to large 60 horsepower AC induction motors. The open-loop VSD employ the simplest motor control, the so-called Volts per Hertz (V/Hz) method. These are also known as xe2x80x9cscalarxe2x80x9d control to differentiate it from the other closed-loop designs where the V/Hz runs in an open loop without a formal feedback device; however, current and voltage sensing is done for current limiting and slip estimation. The V/Hz VSD does not offer torque control or high torque values at low speeds. The V/Hz method is recognized as a lower cost approach to basic motor speed control providing relatively low speed and torque response. It has the advantage to easily control several motors from one drive, popular for driving pumps, fans and other continuous process applications.
FIG. 1 shows a simplified block diagram 100 of a closed loop type variable speed drive and motor. The motor speed controller 101 accepts speed commands form a speed reference 102 and power 104 to be managed by the motor controller to the motor 103. A tachometer 105 is shown to generate the feedback information to close the control loop where the tachometer input shaft is mechanically attached to the rotor of the motor to generate electrical feedback signals to the controller. The tachometer shown represents old technology where today""s designs use an encoder. Motor control designs for the synchronous DC servo motor employ a resolver for feedback information. The speed reference 102 is derived from a 0 to 10 VDC level, typically delivered by a digital to analog converter common to integrated circuits or, for simple manual control, generated by a potentiometer referenced to a VDC source and ground. The input power 104 is selected from a common single phase or three phase (120/208 VAC) bus for compatibility with the basic rating (and motor controller) of the motor. In this approach to VSD, the performance characteristics are much improved in every respect with the additional capacity for torque control. However, the external mechanical encoder requires extra wiring, careful mount alignment with the motor shaft (or geared output) and generally detracts from an intrinsic robustness of the AC motor drive. Moreover, for low fractional horsepower systems, the cost of the sensor can approach 50% of the motor price.
Several manufacturers utilize flux-vector control (FVC) in their closed loop VSD designs. FVC has several variations and is considered the xe2x80x9chigh endxe2x80x9d in induction motor control performance today as opposed to the other methods. The field oriented FVC is the most capable embodiment where it models characteristics of the DC motor through independent control of flux-producing (magnetizing) and torque-producing current components to derive optimal control of motor torque and power. In this version, an actual feedback device, most often an encoder is used for motor position and speed information with very sophisticated motor models used in the control algorithms. FVC allows true torque-mode operation and employs separate speed and torque loops. An adaptive controller adds higher dynamic torque regulation and can account for motor temperature changes and other control disturbances and still deliver optimal torque output. This type of FVC delivers high torque at low speeds and offers very linear parameters over the whole speed range. Other xe2x80x9cvectorxe2x80x9d drives also treat flux and torque currents as a vector sum of total motor current to improve on speed control and torque output but these drive products are derived from a V/Hz base and do not deliver field oriented FVC performance.
The encoderless variable speed drive (EVSD) is the third type of VSD offered today and provides a performance compromise between the expensive closed loop design and the basic V/Hz drives. The EVSD senses the voltage and/or current waveform impressed on the motor drive to motor connection by the running motor to estimate torque and magnetizing components as well as the vector relationship between them. EVSD performance is proportional to the number of motor parameters measured. Without a mechanical feedback device, the EVSD avoids the cost, wear and maintenance problems associated with the closed loop systems; however, the EVSD cannot match the performance of the closed loop VSD. The performance of the three types of variable speed drives available today are shown for comparison in the following table:
Encoderless speed controls or estimators are made based on the principle that the motor speed can be estimated from a measurement of the current and voltage waveform at the motor connection. AC induction motors are driven or speed regulated by a known frequency, the fundamental frequency, and a controlled bus voltage. Motor rotation induces other frequencies in the current waveform due to the physical construction (rotor bars or winding gaps) of the motor rotor. A speed estimate can be generated by detecting these induced speed related frequencies and comparing them to a known mathematical model relating the induced frequencies to the motor speed. This mathematical model is described in P. Alger, xe2x80x9cThe Nature of Induction Machinesxe2x80x9d, Gordon and Breach, New York, 1965. Current EVSDs vary in capability and performance, depending on their derivation from either a V/Hz or vector control base. The ability of the EVSD is very dependent on the latest modeling and adaptation methods since they infer rather than sense motor shaft information.
Current EVSD designs exhibit instability below 1% rated speed which puts a limit on the speed range due to the limitations of current technology. Instability at low speed is exasperated by harsh load dynamics (fast start/stop of high inertial loads) and determines the low speed set point of each application generally determined during system commissioning. It is known to be costly and complex to build a system that will run stable to 0.5 Hz where the end performance is a balance of cost and performance. This low speed system requires the use of more sensors, careful matching of the motor to the drive, tuning tasks, and a longer commissioning time. The best encoderless drives add voltage sensing to current sensing to help with low speed operation.
A 1,800 RPM base speed, 60 Hz induction motor under sophisticated encoderless drive control is generally limited to about 20 RPM. This corresponds to the PWM type inverter operating at a fundamental drive frequency of about 1 Hz under a light load. Current EVSDs mainly use the Fast Fourier Transform (FFT) to detect the speed induced frequencies. Using the FFT, the frequency resolution is shown to depend on the number of samples represented in following equation:
RFFT=Fs/Nxe2x80x83xe2x80x83(1)
Where RFFT is the resolution, Fs is the sampling frequency and N is the sample length. The FFT requires one second or more of sampled data to achieve the 1 Hz frequency resolution. Using non-overlapping blocks of sample data, a one second sample collection time would limit the motor speed to be controlled to once per second. Also, the speed estimate gathered would correspond to the motor speed of up to a second ago. This long delay and low rate of speed estimation limits the low speed of the particular application.
Another problem with prior art EVSD designs is the xe2x80x9csmudgingxe2x80x9d effect that can occur when using a long collection time if the speed of the motor changes during the collection period. The energy in the speed waveform can get spread across several frequency bins of the FFT; consequently, their amplitudes are smaller and potentially undetectable. In some cases, the spectral smudging can be substantial enough to prevent extraction of any kind of information for speed estimation from the spectrum.
Another problem with prior art FFT spectral estimation occurs when the fundamental frequency of the controller is not exactly on a frequency measured by the FFT. In this case, the energy from the fundamental (drive) frequency will be spread out among all the frequencies of the FFT in a phenomenon known as spectral leakage. The spectral leakage of the fundamental frequency may have more energy than those of the speed related frequency information and completely mask the speed related frequency information rendering detection for speed estimation improbable.
In these respects, the present inventive solution substantially departs from the conventional concepts, methods and apparatus designs of the prior art, and in so doing provides a method to provide a speed estimator to enable an encoderless variable speed motor drive with a greater speed range with enhanced stability at low speeds.
In view of the foregoing disadvantages inherent in the known types of motor controller speed estimators now present in the prior art, the present invention provides a method for a speed estimator to estimate the speed of an AC motor to sufficiently signal an encoderless motor controller for rotational stability at all speeds, including slow speeds. The inventive speed estimation method is based on a modified application of the Fast Orthogonal Search (FOS) technique with fast correlation calculations. The inventive approach uses a fraction of the motor waveform samples required with the prior art FFT methods to achieve a faster real-time speed estimate. The shortened computational time of the speed estimator provides for increased torque and rotational stability throughout the speed range, from near zero to the maximum (base) motor speed.
The general purpose of the present invention, which will be described subsequently in greater detail, is to provide an motor speed estimator for an encoderless AC induction motor controller that has many of the advantages of the standard systems mentioned heretofore and many novel features that result in a new method which is not anticipated, rendered obvious, suggested, or even implied by any of the prior art standard motor controllers, either alone or in any combination thereof.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the invention that will be described hereinafter.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting.
A primary object of the present invention is to provide a method that will overcome the shortcomings of the prior art devices.
An object of the present invention is to provide an encoderless speed motor control system with a fast orthogonal search speed estimator and method thereof for estimating the speed of a motor.
Another object of the present invention is to provide a real-time fast orthogonal search speed estimator and method thereof for accurately determining the speed of a motor.
Another object of the present invention is to provide a real-time fast orthogonal search speed estimator and method thereof for determining the speed of a motor that is capable of providing high resolution for motor stability at near zero motor speeds.
Another object of the present invention is to provide a real-time fast orthogonal search speed estimator and method thereof for determining the speed of a motor that is adaptable to current encoderless variable AC motor drive configurations.
Other objects and advantages of the present invention will become obvious to the reader and it is intended that these objects and advantages are within the scope of the present invention. To the accomplishment of the above and related objects, this invention may be embodied in the form illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated and described within the scope of the appended claims.